New research document covering parallel extraction architecture:
rayon page-level parallelism, Arc<> shared xref/font/object-stream
caches, RwLock font cache design, Tesseract thread-local OCR pool,
semaphore memory budget, ordered NDJSON streaming slot array, and
catch_unwind error isolation per page.
Also adds docs/research-index.md: a 622-line navigable index of all
83 research documents grouped into 9 thematic categories, with a
"Start Here" reading path, per-phase implementation reading tables,
and an alphabetical lookup table covering every document.
Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>
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